An air control damper generally comprises a frame dimensioned to fit inside a particular duct, and a series of louver-like blades or leaves that extend across the frame. The blades are pivoted to the frame for movement between an open position in which air can flow along the duct and a closed position in which the duct is effectively closed. Movement of the blades may be accomplished in various ways depending on the event to which the damper is intended to respond.
Air control dampers can be designed to respond to the presence of harmful gasses and/or smoke; such dampers may be designed as smoke dampers, fire dampers or combination smoke and fire dampers. A smoke damper will be typically be designed to close automatically in response to a signal indicating the presence of smoke, for example from a smoke detector. A fire damper will be designed to close automatically in the presence of heat. For example, the blades of the damper may be spring-biassed towards a closed position but held open by a fusible link that melts when exposed to a predetermined temperature. Combination units respond to either smoke or heat.